Near-field optical apparatus is known. See, for instance, D. W. Pohl et al., Applied Physics Letters, Vol. 44(7), p. 651; and U. Durig et al., J. Applied Physics, Vol. 59(10), p. 3318, both incorporated herein by reference. As is well known, critical requirements of near-field optics are radiation source (or aperture) dimensions much less than the wavelength (.lambda..sub.s) of the radiation in air, and a distance between source (or aperture) and object that also is much less than .lambda..sub.s.
The potential of near-field optics for high density data storage has been recognized. See, for instance, E. Betzig et al., Applied Physics Letters, Vol. 61(2), p. 142, which reports the use of near-field scanning optical microscopy to record and image magnetic domains in thin-film magneto-optic materials. Radiation from an Ar.sup.+ laser was coupled into an optical fiber with tapered and metallized probe end. The apparatus facilitated writing of domains of sizes down to about 60 nm, and in the imaging mode attained resolution of 30-50 nm. Data densities of about 7 Gbits/cm.sup.2 were reported. Such high data densities would be highly desirable, if the data could be read out at sufficiently high rates.
A shortcoming of prior art embodiments of near-field imaging is the relatively low attained read-out rate. For instance, the above-cited reference reports a maximum read-out rate of about 10 kHz, limited primarily by shot noise. In view of the rate limitation by shot noise, it is evident that provision of higher photon flux could facilitate higher read-out rates.
E. Betzig et al., Applied Physics Letters, Vol. 63(26), p. 3550, report a near-field imaging experiment that utilized a Nd.sup.3 + doped fiber laser probe with tapered probe end. Operating this fiber laser near its threshold, large modulations in the radiation from the other end of the fiber were observed. However, relaxation oscillations limited the read-out rate to several tens of kHz, and the output of the probe end was only about 50 nW, not sufficient for recording at high data rates.
In view of the potential advantages of near-field optical apparatus, e.g., optical data storage with near-field read and/or write means, it would be highly desirable to have available a readily manufacturable photon source of lateral dimensions much less than .lambda..sub.s that can provide a stable photon flux sufficiently high to permit read-out and/or recording at rates above those achieved by the prior art. This application discloses such a photon source. A. B. Marchant, "Optical Recording", Addison-Wesley 1990, provides background information on optical recording and is incorporated herein by reference.
U.S. Pat. No. 4,860,276 (also incorporated herein by reference) discloses an optical read/write head that utilizes a self-coupled semiconductor laser located close to the surface of the recording medium. Among the disclosed features is provision of an antireflection coating on the laser facet that faces the recording medium, provision of a waveguide lens to focus the laser output on the recording medium, and modifications of the laser to decrease the laser spot size. In particular, the '276 patent discloses a laser 101 as shown in FIG. 1 herein, wherein numerals 102-107 refer, respectively, to substrate, lower clad layer, active layer, upper clad layer, insulation layer and current concentration electrode. Numeral 108 refers to the bottom electrode, and 109 to a cap layer. Numerals 110 and 113 refer, respectively, to the front and rear output end, and 112 refers to a further electrode. Numeral 114 refers to slits for sharpening the beam spot. The slits define a tapered laser region, and extend to substrate 102. They are produced by, e.g., reactive ion etching. Preferably the slits are filled with a filler 115 having lower refractive index than active layer 104. FIGS. 14, 17 and 18 of the '276 patent show modifications of the above-described prior art laser. The modifications also involve slits 114 and filler 115, but have a shaped current concentration electrode 107.
The '276 patent also discloses a monolithic multi-device assembly that comprises an erasing head, a write head and a read head, with longitudinal slits, and with the output end of the erase head being recessed. The '276 patent further discloses (e.g., at col. 9, line 17) that the optical spot size is about 1 .mu.m in diameter, and still further discloses (e.g., col. 8, line 7) an exemplary distance of 2.9 .mu.m between laser front facet and recording medium. The optical head of the '276 patent thus apparently is, strictly speaking, not a near-field device, since the disclosed dimensions are not substantially less than the laser wavelength (exemplarily 830 nm in air; see, for instance, col. 7, line 49).
Lasers with slits as disclosed in the '276 patent are difficult to manufacture, as it is difficult to closely control the lateral spacing between the slits 114 at and near the front facet of the laser; furthermore, it is difficult to reliably manufacture lasers of the prior art type having a spacing between the slits substantially less than .lambda..sub.s, e.g., less than about 0.4 .mu.m, all due to the limitations of photolithography as currently practiced. A further problem in the prior art laser are the relatively high losses in the tapered region, with consequential heating of the laser.
It clearly would be desirable to have available semiconductor lasers for use in near-field optical apparatus that can be manufactured relatively simply to have spot size less than 0.4 .mu.m. The 0.4 .mu.m limit defines the approximate lower spot size limit attainable with photolithography as currently practiced. In preferred lasers disclosed herein, the small spot size is achieved without photolithography.